which means that the actual loss of memory occurred during the night. Further, every case has happened within a circle with a diameter of three hundred miles. We are near the northern edge of that circle."
Carnes checked up on his memory rapidly.
"You're right, Doctor," he cried. "Do you think – ?"
"Once in a while," replied Dr. Bird dryly, "I think enough to know the futility of guesses hazarded without complete data. We are now located within the limits of the amnesia belt and we are here to find out what did happen, if anything, and not to make wild guesses about it. You have the tent set up for us, Major?"
"Yes, Doctor, about thirty yards from the cabin and hidden so well that you could pass it a dozen times a day without suspecting its existence. The gas masks and other equipment which you sent to Fort Banks are in it."
"In that case we had better dispense with your company as soon as we have eaten a bite, and retire to it. On second thought, we will eat in it. Carnes, we will go to our downy couches at once and leave our substitutes in possession of the cabin. I trust, gentlemen, that things come out all right and that you are in no danger."
Major Trowbridge shrugged his heavy shoulders.
"It is as the gods will," he said sententiously. "It is merely a matter of duty to me, you know, and thank God, I have no family to mourn if anything does go wrong. Neither has Corporal Askins."
"Well, good luck at any rate. Will you guide Carnes to the tent and then return here and I'll join him?"
Huddled in the tiny concealed tent, Dr. Bird handed Carnes a haversack on a web strap.
"This is a gas mask," he said. "Put it on your neck and keep it ready for instant use. I have one on and one of us must wear a mask continually while we are here. We'll change off every hour. If the gas used is lethane, as I suspect, we should be able to detect it before its gets too concentrated, but some other gas might be used and we must take no chances. Now look here."
With the aid of a flash-light he showed Carnes a piece of apparatus which had been set up in the tent. It consisted of two telescopic barrels, one fitted with an eye-piece and the other, which was at a wide angle to the first, with an objective glass. Between the two was a covered round disc from which projected a short tube fitted with a protecting lens. This tube was parallel to the telescopic barrel containing the objective lens.
"This is a new thing which I have developed and it is getting its first practical test to-night," he said. "It is a gas detector. It works on the principle of the spectroscope with modifications. From this projector goes out a beam of invisible light and the reflections are gathered and thrown through a prism of the eye-piece. While a spectroscope requires that the substance which it examines be incandescent and throw out visible light rays in order to show the typical spectral lines, this device catches the invisible ultra-violet on a fluorescent screen and analyzes it spectroscopically. Whoever has the mask on must continually search the sky with it and look for the three bright lines which characterize lethane, one at 230, one at 240 and the third at 670 on the illuminated scale. If you see any bright lines in those regions or any other lines that are not continually present, call my attention to it at once. I'll watch for the first hour."
At the end of an hour Dr. Bird removed his mask with a sigh of relief and Carnes took his place at the spectroscope. For half an hour he moved the glass about and then spoke in a guarded tone.
"I don't see any of the lines you told me to look for," he said, "but in the southwest I get wide band at 310 and two lines at about 520."
Dr. Bird advanced toward the instrument but before he reached it, Carnes gave an exclamation.
"There they are, Doctor!" he cried.
Dr. Bird sniffed the air. A faint sweetish odor became apparent and he reached for his gas mask. Slowly his hands drooped and Carnes grasped him and drew the mask over his face. Dr. Bird rallied slightly and feebly drew a bottle from his pocket and sniffed it. In another instant he was shouldering Carnes aside and staring through the spectroscope. Carnes watched him for an instant and then a low whirring noise attracted his attention and he looked up. Silently he caught the Doctor's arm in a viselike grip and pointed.
Hovering above the cabin was a silvery globe, faintly luminous in the moonlight. From its top rose a faint cloud of vapor which circled around the globe and descended toward the earth. The globe hovered like a giant humming bird above the cabin and Carnes barely stifled an exclamation. The door of the cabin opened and Major Trowbridge, walking stiffly and like a man in a dream, appeared. Slowly he advanced for ten yards and stood motionless. The globe moved over him and the bottom unfolded like a lily. Two long arms shot silently down and grasped the motionless figure and drew him up into the heart of the globe. The petals refolded, and silently as a dream the globe shot upward and disappeared.
"Gad! They lost no time!" commented Dr. Bird. "Come on, Carnes, run for your life, or rather, for Trowbridge's life. No, you idiot, leave your gas mask on. I'll take the spectroscope; it'll be all we need."
Followed by the panting Carnes, Dr. Bird sped through the night along an almost invisible path. For half a mile he kept up a headlong pace until Carnes could feel his heart pounding as though it would burst his ribs. The pair debouched from the trees into a glade a few acres in extent and Dr. Bird paused and whistled softly. An answering whistle came from a few yards away and a figure rose in the darkness as they approached.
"Maynard?" called Dr. Bird. "Good enough! I was afraid that you might not have kept your gas mask on."
"My orders were to keep it on, sir," replied the lieutenant in muffled tones through his mask, "but my mechanician did not obey orders. He passed out cold without any warning about fifteen minutes ago."
"Where's your ship?"
"Right over here, sir."
"We'll take off at once. Your craft is equipped with a Bird silencer?"
"Yes, sir."
"Come on, Carnes, we're going to follow that globe. Take the front cockpit alone, Maynard; Carnes and I will get in the rear pit with the spec and guide you. You can take off your gas mask at an elevation of a thousand feet. You have pack 'chutes, haven't you?"
"In the rear pit, Doctor."
"Put one on, Carnes, and climb in. I've got to get this spec set up before he gets too high."
The Douglass equipped with the Bird silencer, took the air noiselessly and rapidly gained elevation under the urging of the pilot. Dr. Bird clamped the gas-detecting spectroscope on the front of his cockpit and peered through it.
"Southwest, at about a thousand more elevation," he directed.
"Right!" replied the pilot as he turned the nose of his plane in the indicated direction and began to climb. For an hour and a half the plane flew noiselessly through the night.
"Bald Mountain," said the pilot, pointing. "The Canadian Border is only a few miles away."
"If they've crossed the Border, we're sunk," replied the doctor. "The trail leads straight ahead."
For a few minutes they continued their flight toward the Canadian Border and then Dr. Bird spoke.
"Swing south," he directed, "and drop a thousand feet and come back."
The
